1. Field of the Invention
The present invention relates to a multi-ring network structured by connecting a plurality of rings having different transmission speeds by nodes.
2. Description of the Related Art
In connecting subscribers' terminals to a network, a star network may be used if the subscribers' terminals are located relatively close to each other. However, when terminals are spaced relatively far apart, a ring network such as a local area network (LAN) will be effectively used. As an example of a network having such spaced apart terminals, there is an emergency telephone network run by a public institution. The telephones connected to this network are such that they are installed along a highway or the like at regular intervals. Such an emergency telephone system has a great importance sometimes relating to people's lives. Therefore, it is required to enhance the reliability on the line as much as possible and hence the line is required to be provided in a duplicative structure.
A conventional ring network in general use is shown in FIG. 1. A plurality of nodes A', B', C', and D' are connected by a transmission line 1 drawn by a solid line and a backup transmission line 2 drawn by a broken line, in which arbitrary nodes B' and C' are connected with terminals 3 and 4. Since the nodes are connected by a pair of transmission lines 1 and 2 to form a ring, the transmission line has a duplicative structure.
When, in general, communication is to be made between the terminals 3 and 4, data output from the terminal 3 is transmitted through the transmission line 1 counterclockwise to be input to the terminal 4. Data output from the terminal 4 is also transmitted through the transmission line 1 counterclockwise to be input to the terminal 3. When a line breakage occurs somewhere in the transmission line 1, the backup transmission line 2 is put into use. In this case, data is transmitted clockwise.
Referring to FIG. 2, there is shown a schematic diagram illustrating a case where communication is established between two ring networks. When the ring network 5 and the ring network 6 communicate with each other, the node 8 of the ring network 5 and the node 9 of the ring network 6 are connected by a dedicated transmission line 7.
However, when the two ring networks 5 and 6 are connected by a dedicated transmission line 7 as shown in FIG. 2, it is impossible to make the transmission line 7 duplicative. Accordingly, when trouble such as a line breakage occurs in the transmission line 7, the communication between the ring network 5 and the ring network 6 becomes interrupted. Thus, there has been a problem of low reliability on the network-to-network communication.